Validate base64 embedding inputs use data URI format (#145038)

The EMBEDDING task type requires that image inputs using the BASE64
format are data URIs. This commit adds validation that inputs using the
BASE64 format conform to the data URI format; i.e. that they begin
with "data:{some text}/{some other text};base64,"

This validation does not guarantee that the specified MIME type is
valid, that the MIME type matches the base64 data, or that the base46
data in the input is valid.